The present invention relates to a method of preparing a pressure-sensitive adhesive comprising providing a coatable partially polymerized radiation-curable precursor of the adhesive and applying the precursor to a substrate with subsequent further polymerization.
Pressure-sensitive adhesives can be applied to a substrate as pre-cured adhesive tapes which may be die-cut to the required shape. While this technique is convenient for many technical applications and does, in particular, usually not require any in-situ curing step, it may be disadvantageous in other technical applications for the following reasons:                Double-coated adhesive tapes usually require the use of one or more release liners which have to be discarded upon application of the adhesive tape. When die-cutting the adhesive tape to the desired shape, the excess tape has to be discarded as well.        Especially in the electronics industry the shapes of the substrates to be adhered tend to get smaller and more complicated so that the die-cutting of the adhesive tape tends to become more difficult and technically demanding.        
U.S. Pat. No. 4,181,752 discloses a process for the preparation of a normally tacky pressure-sensitive adhesive comprising subjecting a suitable radiation-sensitized solventless liquid mixture containing as a major proportion thereof at least one acrylic acid ester of an alkyl alcohol containing from 4-14 carbon atoms, and at least one monomer copolymerizing therewith to prepolymerizing conditions to form a relatively viscous partially polymerized mixture or syrup in which about 5-10% of the monomers have been converted to polymers, applying said partially polymerized mixture to a substrate in a thickness of up to about 1.9 cm in thickness and subjecting it to radiation in the near UV-region at a rate of from 0.1-7 mW/cm2 of the partially polymerized mixture exposed.
The further polymerization of the partially polymerized mixture of U.S. Pat. No. 4,181,752 is carried out in an inert atmosphere such as N2, CO2, He or Ar which is disadvantageous for various practical applications. The method of forming the partially polymerized mixture used in U.S. Pat. No. 4,181,752 provide a coatable viscosity of the partially polymerized mixture only at low degrees of conversion of the monomers to polymers of typically less than about 10%. Due to the low degree of conversion of the monomers to polymer the further polymerization of the partially polymerized mixture requires the application of UV-irradiation with a relatively high energy density which may damage, for example, a substrate the partially polymerized mixture is applied to. If the degree of conversion of the monomers to polymers is increased, the viscosity of the partially polymerized mixture tends to become very high rendering the mixture non-coatable. It was also found that if the degree of conversion of monomers to polymer is increased, the resulting partially polymerized mixture tends to gel and/or become inhomogeneous.
It was a desire of the present invention to provide a method of preparing a pressure-sensitive adhesive comprising the preparation of a partially pre-polymerized coatable radiation-curable precursor with subsequent further polymerization to a pressure-sensitive adhesive with advantageous properties which does not exhibit the drawbacks of previously known methods or exhibits such drawbacks to a limited degree only. It was another desire of the present invention to provide a coatable radiation-curable precursor of a pressure-sensitive adhesive which can be polymerized in a non-inert atmosphere.